As electronic circuits become more and more miniaturized, the density of heat-producing circuit elements increases, requiring greater and greater heat conductivity in the ceramic substrate to dissipate this heat and prevent it from interfering with the normal operation of the circuits. For some applications, the conventional aluminum oxide substrates have inadequate heat conductivity. It is known to solve this problem by substituting beryllium oxide, which has an eight-fold better heat conductivity, for the aluminum oxide, but this is considerably more expensive. It has been found that a particular problem is the extreme toxicity of beryllium oxide dust, which practically precludes subsequent processing steps by the user such as grinding or polishing. Finishing must be performed in specially monitored work stations. U.S. Pat. No. 3,813,252 discloses similar sintered compositions.